The method of the present invention relates to coarse tuning a digital radiotelephone's channel frequency receiver's down converter frequency error when the telephone switches from one channel to another.
This problem is particularly emphasized when a dual-mode analog/digital telephone, e.g., a dual-mode radio telephone used in the United States (USDMR), is switched from the analog channel to the digital channel. In the analog state (e.g., using the AMPS system) the frequency stability requirement of the base station is 1.5 ppm, and the requirement on the telephone is 2.5 ppm. Correspondingly, in the digital state the base station frequency stability requirement is 0.25 ppm and on the mobile station it is .+-.200 Hz. Thus, the frequency error when the telephone operates on the digital channel after switching over from the analog channel could be as high as 2500+250 Hz. A frequency error of this magnitude means a considerable phase error for each symbol, depending on the symbol rate. In the above case, at the USDMR system symbol rate of 24,300 baud, the phase error would be more than 40 degrees per symbol, when the system utilizes differentially encoded quadrature amplitude modulation (QAM). The frequency error must be reduced in order to synchronize the receiver to the incoming data.